1. Field of Disclosure
This disclosure relates generally to methods and apparatus for printing and dispensing viscous material, such as solder paste, onto a substrate, such as a printed circuit board, and more particularly to a combination stencil printer and dispenser and related methods of printing and dispensing.
2. Discussion of Related Art
In a typical surface-mount circuit board manufacturing operation, a stencil printer is used to print solder paste onto a printed circuit board. A circuit board, broadly referred to as an electronic substrate, having a pattern of pads or some other conductive surface onto which solder paste will be deposited, is automatically fed into the stencil printer. One or more small holes or marks on the circuit board, called fiducials, is used to align the circuit board with the stencil or screen of the stencil printer prior to the printing of solder paste onto the circuit board. The fiducials serve as reference points when aligning a circuit board to the stencil. Once a circuit board has been aligned with the stencil in the printer, the circuit board is raised to the stencil by a substrate support, e.g., a table having pins or other work holders. Solder paste is then dispensed by moving a wiper blade or squeegee across the stencil to force the solder paste through apertures formed in the stencil and onto the board. As the squeegee is moved across the stencil, the solder paste tends to roll in front of the blade, which desirably causes mixing and shearing of the solder paste so as to attain a desired viscosity to facilitate filling of the apertures in the screen or stencil. The solder paste is typically dispensed onto the stencil from a standard solder paste supply cartridge. After the print operation, the board is then released, lowered away from the stencil, and transported to another station within the printed circuit board fabrication line. Specifically, the stencil is separated from the board and the adhesion between the board and the solder paste causes most of the material to stay on the board. Material left on the surface of the stencil is removed in a cleaning process before additional circuit boards are printed. In some applications, a cleaning assembly is moved under the stationary stencil and excess material is wiped off of the stencil as the cleaning assembly is moved. In another application, the stencil is moved over the stationary cleaning assembly to clean the underneath of the stencil.
The manufacturing of circuit boards involves many processes, one of which is the screen printing of solder paste (or other adhesives or materials) on the surface of a circuit board with the stencil printer so that electronic components can thereafter be deposited onto the board, which is described above. Another process is to dispense metered amounts of materials (liquid or paste) for a variety of applications with a separate dispenser. One such application is the assembly of integrated circuit chips and other electronic components onto circuit board substrates. In this application, automated dispensing systems are used for dispensing dots of liquid epoxy or solder paste, or some other related material, onto circuit boards. Automated dispensing systems are also used for dispensing lines of underfill materials and encapsulents, which mechanically secure components to the circuit board. Underfill materials and encapsulents are used to improve the mechanical and environmental characteristics of the assembly. Another application is to dispense very small amounts or dots onto a circuit board. Others include a syringe that utilizes pressure to dispense material from the syringe. In one system capable of dispensing dots of material, a dispensing pump utilizes a rotating auger having a helical groove to force material out of a nozzle and onto a circuit board. Yet another application is to dispense dots of material by using a fetter.
Another process in the printing of circuit boards involves inspection of the boards after solder paste has been deposited on the surface of the boards. Inspecting the boards is important for determining that clean electrical connections can be made. An excess of solder paste can lead to shorts, while too little solder paste in appropriate positions can prevent electrical contact. Generally, in a stencil printer, a vision inspection system is employed to provide a two-dimensional or a three-dimensional inspection of the solder paste on the board.
Stencil printers typically contain an individual personal computer or “PC” and the only standard level of communication to adjacent pieces of equipment is through an established protocol. For example, a printed circuit board fabrication line may include one or more pieces of equipment, such as stencil printers, dispensers, pick-and-place machines, reflow ovens, wave soldering machines and/or inspection machines. Typically, process lines including stencil printers and placement machines are operated by a pull system with the placement machines controlling the introduction of product into different pieces of equipment in the production line. This is typically done in an attempt to maximize cycle time performance, primarily of the placement equipment. The production line is usually designed around the placement machines. The placement machines are typically the more expensive equipment in the production line, and so effort is made to keep them operating at maximum capacity. Errors discovered in the printing process may require manual intervention to rectify, which can impact the flow of materials into the placement system, decreasing the utilization of the placement system.